It is well known to (co)polymerize ethylene and .alpha.-olefin in the presence of a metallocene/aluminoxane catalyst system. The first very active catalytic system of this type that was discovered is that based on zirconocene: Cp.sub.2 ZrCl.sub.2 /aluminoxane. Metallocene/aluminoxane catalyst systems are soluble in the polymerization medium. The extension of research in this field has led to the discovery of other metallocene catalysts, such as bridged metallocenes which are capable, in the case of the copolymerization of ethylene with .alpha.-olefins, of leading to better uniformity in the distribution of the comonomers in the molecular chains.
However, aluminoxanes, in particular methylaluminoxane which is the most commonly used, have the drawbacks of being expensive and unstable substances which are partly responsible for the poor morphology of the polymers, a situation which causes fouling of the reactors and which makes the conveying process very complicated.